Long-term dynamics of phosphorus concentration and assessment of eutrophication in the lower reaches of the Don River

Le résumé fourni par la source

Background. Excessive input of biogenic substances into surface waters poses an environmental problem. In the late 20th – early 21st centuries, biogenic load to rivers decreased due to activities related to water resource management (wastewater treatment, ban on the use of phosphate detergents, reduction in the use of some agricultural fertilizers, etc.). Relevance. Reliable data on phosphorus levels in water and their long-term dynamics in surface waters are scarce, mainly due to data quality issues. There is still little information on how the trophic status of the lower Don River has changed. The aim of this work is to conduct the comparative analysis of long-term (1986–2002 and 2011–2020) trends in changes of mineral and total phosphorus levels in the lower reaches of the Don River and to assess the related change in the probabilities of development of certain trophic conditions. Methods. This study has been carried out based on the data obtained through the analysis of phosphate and total phosphorus content in water in the outlet section of the Don River (stanitsa Razdorskaya) in 1986–2002 and 2011–2020 and the further statistical modeling using the WRTDS (Weighted Regressions on Time, Discharge, and Season) method. Results. Major trends in variation of the levels of phosphates and total phosphorus have been identified. The concentration of phosphates and total phosphorus shows negative dependence on water discharge. Since 1995, there has been a decreasing trend in the relative share of the mineral phosphorus component in total phosphorus and an increasing trend for the organic component. Based on the levels of total phosphorus in the water, the trophic status of the lower Don River has been defined for the period 1986–2020. The status of the lower Don River changed from mesotrophic in the 1990s to eutrophic in the first decade of the 21st century. In the second decade of the 21st century, a positive trend in phosphorus levels in the river water changed to a negative one, and in 2015, the eutrophic conditions shifted to mesotrophic. Conclusion. The mechanisms responsible for the observed long-term changes in phosphorus levels in the lower reaches of the Don River remain unclear and may be clarified by further studies. We believe that the observed long-term variations in phosphorus levels in the river water are related to drastic changes that have been occurring in agriculture, industry, and other economic activities since 1991. Changes in the hydrological regime (low river flow) have also contributed by affecting the qualitative characteristics of the runoff of biogenic elements and their ratios.

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